Since the advent of the larger size resealable plastic 1 liter and 2 liter bottles the problem has existed that if the pop is not all used and the bottle is resealed, the carbon dioxide in the liquid comes out of solution and builds pressure in the bottle until the partial pressure of the remaining dissolved gas is equal to the pressure of the gas in the empty space in the bottle. Further, each time the bottle is reopened the gas pressure that has built up in the bottle is lost in the atmosphere and must rebuild after reclosure by further loss of dissolved gas. As a 2 liter bottle holds 8 to 10 medium sized glasses of liquid, the bottle may need resealed up to 10 times or once for each glass poured, thereby losing almost all of the dissolved carbon dioxide fizz by the time the last glass is to be used
Obviously there are ways to repressurize the bottle as it was pressurized originally, but to date there exists no very low cost approach or device for the average consumer to use to keep the pop in the bottle from going flat (i.e. loosing all the "fizz") after opening and reopening the bottle.